Process of making magnesium oxide cores

ABSTRACT

Magnesium oxide is utilized as a water soluble core for producing hollow superalloy parts by investment casting techniques. The core can be formed by mixing slurries of magnesium oxide and magnesium oxychloride, allowing the mixture to harden and then firing the body to form the corresponding oxide.

United States Patent 1 Anderson et a1.

1541 PROCESS OF MAKING MAGNESIUM OXIDE CORES [75] Inventors: John R. Anderson, Bloomfield, N.J.;

James S. Perron, Deep River, Conn.

[73] Assignee: United Aircraft Corporation, East Hartford, Conn.

22 Filed: June29, 1971 21 Appl.No.: 158,128

[52] US. Cl. ..164/4l, 164/72, 164/132, 106/383 [51] Int. Cl ..B22c 1/02 [58] Field of Search .....164/4l, 42, 132;167/72, 138

[5 6] References Cited UNITED STATES PATENTS 3,643,728 2/1972 Hulse ..164/l32 1 1 Mar. 27, 1973 Rose ..l64/132 X Anderko ..164/41 FOREIGN PATENTS OR APPLICATIONS 1,306,572 9/1962 France 164/41 Primary Examiner-J. Spencer Overholser Assistant Examiner-John E. Roethel Attorney-Richard N. James 57 ABSTRACT 3 Claims, No Drawings PROCESS OF MAKING MAGNESIUM OXIDE CORES BACKGROUND OF THE INVENTION 1 Field of the Invention The present invention relates to the production of hollow, superalloy parts by investment casting techniques and more particularly to the production of metal parts formed of cobalt or nickel superalloys and containing small and narrow apertures and passages in the castings.

2. Description of the Prior Art In investment casting, a disposable pattern, which is a replica of the part to be cast and which includes any necessary gates and risers, is usually dipped in a refractory slurry which hardens to form a smooth coating that serves as the mold face. The precoating or dipcoating of the pattern usually is carried out by immersing the pattern in a suspension of a fine refractory powder in a suitable liquid binder that is capable of hardening during drying at room conditions. Following dipping, the excess slurry is drained from the pattern and the coating is stuccoed while wet with coarser refractory particles which help to set the coating.

A ceramic shell mold is usually prepared by repeating the dipping and stuccoing operations described above until a shell having a sufficient thickness to resist the stresses occuring in subsequent operations is built up around the pattern. The usual thickness range is from one-eighth of an inch to one-half of an inch, although thinner or heavier shells may be formed for special situations. After forming the refractory shell, the disposable pattern is removed from the mold which is then prepared for the casting operation. The typical pattern materials used in the process have been either wax blends or plastics such as polystyrene and, occasionally, polyethylene.

It is sometimes desirable to form hollow articles and particularly ones with very small and narrow passages by these investment casting techniques. Commonly, to make such articles, a ceramic core, usually formed of pressed silica, is disposed within the mold at an appropriate location. While these silica cores are quite useful, they are difficult to remove with a non-corrosive leaching media after the casting has been made. It can be appreciated that when very small or fine apertures or interstices are filled with an insoluble core material that difficulty can be encountered when attempting to totally remove the material with a non-corrosive leaching medium.

Other cores used for forming hollow cast articles have included soluble metal halide salts mixed with borax, magnesium oxide or talc. Such compositions have been disclosed in the United States Pat. to Anderko, No. 3,407,864. Rose discloses in the United States Pat. No. 3,473,599, the use of a mixture of magnesium oxide and calcium phosphate. In the case of the halide-containing core, we have found that the halogen can react undesirably with the superalloy during casting. In the case of the magnesium oxide-calcium phosphate core, we have found that it is gassy, brittle and difficult to use when making castings of superalloys.

SUMNIARY OF THE INVENTION According to the present invention we have discovered a method of successfully producing ceramic cores for investment casting processes. These cores, made of magnesium oxide, can easily withstand the temperatures encountered when casting superalloys, do not react with the alloy systems and can be easily formed into complex shapes. Quite advantageously, magnesium oxide, formed according to the present process, can be removed easily from the casting through the use of non-corrosive leaching medium, particularly water.

In the present process an aqueous solution of 12 to 30 w/o magnesium chloride is mixed with w/o magnesium oxide powder to form a thick slurry. The slurry is poured into a plastic or rubber die having the configuration which is desired. The components react to form a complex magnesium oxychloride cement which is relatively rigid, even in the unfired state. When solid, the body is fired to convert the oxychloride to the oxide, generally at temperaturesbetween about 2,200 to 2,700 F. A solid, rugged magnesium oxide core is produced which can be subsequently incorporated in an investment casting process.

The core which was formed is generally coated by dipping it in a slurry of finely divided zirconium oxide (having an average particle size below. microns) to prevent the reaction of the magnesium with any silicates from the exterior shell of the investment casting. After the casting is made according to conventional techniques, the core of magnesium oxide may be removed by playing a jet of live steam upon it or placing the part in an autoclave filled with water at a relatively high temperature. The magnesium oxide dissolves in the steam and other leaching agents need not be added for its removal. When the core is removed from the casting, the apertures or interstices are disposed within it.

As a specific example of the present invention, we dissolved 167 grams of magnesium chloride (MgCl 6H O) in 100 grams water to form a solution. To this solution grams of magnesium oxide were added to 55 grams of solution and a magnesium oxychloride cement (plus MgO) was formed. This cement was poured into a plastic die to solidify. When solid, it was removed from the die and fired at a temperature of 2,700F. to convert the oxychloride to the oxide.

It is apparent that modifications and changes may be made within the spirit and scope of the present invention but it is our intention, however, only to be limited by the scope of the appended claims.

As our invention, we claim:

1. A method of forming a water-soluble core for use in the manufacture of hollow articles in investment casting processes, said core being adapted to be disposed within the casting so that, upon removal, hollow passages remain, said method including the steps of: admixing an aqueous solution of magnesium chloride with magnesium oxide powder to form a slurry; pouring said slurry into a die of the desired shape and allowing said slurry to harden to form a body corresponding to the shape of said die; thing said body at a temperature of 2,2002,70()F. to oxidize said magnesium chloride and form a solid magnesium oxide core, said core being useful in said investment casting process.

2. The method according to claim 1 wherein said slurry contains between about 12 and 30 w/o magnesium chloride and 60 and 80 w/o magnesium oxide.

3. A method of forming a water-soluble core for use in the manufacture of hollow articles in investment casting processes, said core being adapted to be I disposed within the casting so that, upon removal, hollow passages remain, said method including the steps of: admixing an aqueous solution of magnesium chloride with magnesium oxide powder to form a slur- 

2. The method according to claim 1 wherein said slurry contains between about 12 and 30 w/o magnesium chloride and 60 and 80 w/o magnesium oxide.
 3. A method of forming a water-soluble core for use in the manufacture of hollow articles in investment casting processes, said core being adapted to be disposed within the casting so that, upon removal, hollow passages remain, said method including the steps of: admixing an aqueous solution of magnesium chloride with magnesium oxide powder to form a slurry; pouring said slurry into a die of the desired shape and allowing said slurry to harden to form a body corresponding to the shape of said dIe; firing said body to oxidize said magnesium chloride and form a solid magnesium oxide core, and coating the core with zirconium oxide having a particle size less than about 100 microns, the coated core being useful in said investment casting process. 